Giant telescope in race to become world's largest

The Giant Magellan Telescope, which is in a race to become the world's largest telescope, will be built in central Chile, officials announced on Thursday. Set to be completed in 2016, the GMT will be able to produce images up to 10 times as sharp as the Hubble Space Telescope.

After considering various sites in northern Chile, telescope officials decided to build the GMT in central Chile at the mountaintop site of Las Campanas Observatory, the existing home of the twin 6.5-metre Magellan Telescopes. The observatory is located in Chile's Atacama Desert and boasts a dry climate, dark skies and a stable atmosphere - all key conditions for making good astronomical observations.

"Excellent science has come from Las Campanas for several decades," says Wendy Freedman, head of the GMT Board and director of the Carnegie Observatories, which operates Las Campanas. "The superb astronomical quality of the site is a significant contributor to this success."

The GMT will use seven 8.4-metre primary mirrors that will be arranged into a single "super dish" with one mirror at the centre and the other six curved around it like petals. Such "off-axis" mirrors have never been made as large as this. The design makes the telescope's vision keener than it would be if all seven mirrors remained separate, and it will be able to resolve details about 10 times sharper than Hubble.

'New age'

The GMT will have an effective aperture of 24.5 metres, much larger than the twin 10-metre Keck telescopes in Hawaii, US, which are currently the world's largest optical telescopes. "We hope to have four of the . . . mirrors in the telescope mount by the year 2013, at which time this would be the world's largest telescope," Freedman told New Scientist. The GMT is scheduled to be finished by 2016.

But it's not certain whether - or how long - the GMT would actually hold the title, since even larger telescopes are currently being planned for the next decade, such as the Thirty Meter Telescope, and a 42-metre telescope called the European Extremely Large Telescope.

"It's not for sure what order these will all be built in," says Arnold Phifer, director of external affairs at the Carnegie Observatories. "[But] it is larger than any telescope ever built."

Charles Alcock, director of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, a member of the GMT consortium, says these giant eyes on the sky will usher in "a new age of astronomical exploration. As telescopes get larger, we are able to see fainter, farther and with more clarity than ever before".

High polish

Astronomers hope to study extrasolar planetary systems, the formation of stars, galaxies and black holes and the nature of dark matter and dark energy with the telescope.

The GMT's first mirror - made from 18 tonnes of borosilicate glass made from sand gathered in Florida, US - was cast at the University of Arizona's famed Mirror Laboratory in 2005.

Telescope officials decided to build one of the six off-axis mirrors first, since they - unlike the central mirror - cannot be perfectly symmetric in their concavity. "The six are higher and thicker on the outside [of the super dish] and narrower and thinner on the inside," says Phifer. "We wanted to prove to the astronomical community and potential funders that we're sure we know all the parameters of [how to build it]."

That first mirror is now being polished - a painstaking process that is likely to take until 2009. When it is completed, its surface will be extremely smooth, containing bumps or dips no taller than 25 nanometres.

The entire project is estimated to cost $550 million, and so far about $25 million to $35 million has been raised, Phifer says.

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The Giant Magellan Telescope will use seven 8.4-metre mirrors arranged into a 'super dish' with an effective aperture of 24.5 metres (Illustration: Giant Magellan Telescope/Carnegie Observatories)